OS X: It's easy to run out of disk space when you're not paying attention, and often times it's because you don't empty the trash. Empty is an app that reminds you when your trash gets excessively full. You can just set a warning limit and it'll let you know.

The app lives in your Mac's menu bar and you can adjust settings whenever you like. It only does this one little thing, but if you have trouble remembering to keep your computer clean it can help.